CN111521507B - Test method for thermal fatigue surface crack propagation rate of environment-friendly stainless steel weld joint - Google Patents
Test method for thermal fatigue surface crack propagation rate of environment-friendly stainless steel weld joint Download PDFInfo
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Abstract
The invention discloses a test method of thermal fatigue surface crack propagation rate of an environment-friendly stainless steel welding seam, which is used for carrying out thermal fatigue simulation on a working condition that the environment-friendly stainless steel welding seam is in a cold and hot load needing to experience large temperature difference for many times, and adding rare earth elements into a thermal fatigue sample before welding to obtain the conclusion that the rare earth elements are added during welding of the environment-friendly stainless steel to obtain good thermal fatigue resistance; for the thermal fatigue surface crack propagation curve of the ferritic stainless steel welded joint, the fitting precision of the thermal fatigue surface crack propagation curve obtained by selecting an exponential model is higher, and the test data is more accurate; the surface crack propagation rate obtained by calculation of the propagation rate equation after the thermal fatigue crack propagates and enters the stable propagation stage is consistent with the actual test, and reliable data support can be provided for the actual production.
Description
Technical Field
The invention relates to a weld joint test method, in particular to a weld joint thermal fatigue surface crack propagation rate test method suitable for a resource-saving environment-friendly stainless steel joint, and belongs to the technical field of welding.
Background
The chromium content of the ferritic stainless steel (400 series) is 15-30 percent, and the ferritic stainless steel has a body-centered cubic crystal structure. The steel does not generally contain nickel, sometimes contains a small amount of Mo, ti, nb and other elements, has the characteristics of large heat conductivity coefficient, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance and the like, and is mainly used for manufacturing parts which are resistant to corrosion of atmosphere, water vapor, water and oxidizing acid. Since ferritic stainless steels contain little or no nickel, they are known in the industry as resource-saving, environmentally friendly materials.
409L is a typical representative of low-chromium-nickel ferritic stainless steel and is commonly used in the manufacture of automobile exhaust manifolds, however, the temperature of the manifold can reach 800-1000 ℃ in the working process, and the manifold welding joint needs to experience cold and hot loads with large temperature difference for many times along with the continuous start or stop of an automobile, so that the thermal fatigue surface cracks and even the fracture are generated at the welding joint of the manifold welding joint, and therefore, the research on the thermal fatigue characteristics has practical significance.
At present, in the research on fatigue crack propagation, the endurance limit of stainless steel at constant temperature is researched by using an S-N curve, and the relation between the crack length and the cycle number is researched by using an a-N curve graph obtained by a finite element and a rain flow meter algorithm. But fatigue reliability was studied on the S-N curve with a certain probability level. To obtain a more accurate S-N curve, it is often necessary to adjust the existing material parameters or to develop a new fatigue model. The a-N curve and the S-N curve can be used for researching fatigue reliability, the a-N curve and the S-N curve have certain compatibility when the fatigue reliability of a welding joint is researched, and research finds that a crack propagation rate curve corresponding to initial damage at the front edge of a crack can be obtained through parallel movement of the S-N curve in a dual-logarithm diagram. And the factors influencing the fatigue crack propagation characteristics are more and more complex, so that the main influencing factors are grasped in the research, and the test variables are controlled.
In the case of tubular structures such as automotive exhaust pipes, surface cracks are the most common defects in welded tubular joints and are the focus of research in calculating fatigue of such structural members. In the research of the surface crack propagation rate, the calculation of the stress intensity factor of the surface crack of the pipe node is more important. The stress intensity factors of the cracks of the T-shaped plate nodes, the Y-shaped plate nodes and the plate surfaces are calculated conventionally, the influences of residual stress, crack propagation angles, stress redistribution and load effects are not considered, and the problem of establishing a new model of the stress intensity factors suitable for the cracks of the surfaces of the plate nodes and the tubular nodes becomes a research hotspot. The method is characterized in that external load influence factors are abandoned, thermal deformation, metal thermal activation recovery, dynamic recovery and recrystallization of a welding joint under a high-temperature condition, which are caused by different materials at the welding joint, may influence the structure refinement, dislocation density, strain rate, crack propagation rate and propagation mechanism of the welding joint, which are important factors influencing the fatigue life of the welding joint, but the failure caused by crack propagation is relatively more, so the research on the crack propagation rate of the welding joint is the key point of the research on the thermal fatigue characteristic, but at present, related research on the crack propagation rate of the welding joint of the environment-friendly stainless steel joint is rarely recorded, and the data of the crack propagation rate of the surface of the traditional stainless steel often does not conform to the actual production.
Disclosure of Invention
Aiming at the problems, the invention provides a test method for the thermal fatigue surface crack propagation rate of an environment-friendly stainless steel weld joint, the test method is used for obtaining an environment-friendly stainless steel weld joint thermal fatigue surface crack propagation rate equation, the surface crack propagation rate calculated by the equation is in accordance with the actual test, reliable data support can be provided for the actual production, and the test method is particularly suitable for the working condition that 409L low-chromium-nickel ferrite stainless steel is applied to the cold and hot load needing to experience large temperature difference for many times.
In order to achieve the purpose, the test method for the thermal fatigue surface crack propagation rate of the environment-friendly stainless steel weld joint specifically comprises the following steps:
step one, sample preparation: the austenitic stainless steel is used as a filling material, two ferritic stainless steel samples are cut after butt welding, the center of a cutting processing welding line is enabled to coincide with the center of a V-shaped notch of the sample, and a thermal fatigue sample containing the V-shaped notch, which is required by a test, is obtained;
step two, thermal fatigue test: performing thermal fatigue test on the thermal fatigue test sample according to the set test times on a thermal fatigue testing machine, and measuring and recording the surface crack of the thermal fatigue test sample when the set test time data sampling point is reached;
step three, calculating the thermal fatigue surface crack propagation rate of the welding joint: selection of an exponential model
Wherein C is 1 、C 2 、C 3 、C 4 Taking the crack length a as the crack length value of a fitting curve corresponding to a single test index, taking the cycle number N as the cycle number value corresponding to a single test variable, and fitting a plurality of surface crack length data points acquired by test number data sampling points by using Matlab software to obtain an a-N surface crack propagation curve equation a =206.4e 0.00293N -260.1e -0.01119N ；
Introduces Paris formula da/dN = C (delta K) m Solving the expansion rate of the thermal fatigue surface crack in the stable expansion stage of the parent metal sampleThe formula is shown in the specification, wherein a represents the crack length, N represents the cycle number, C and m are material parameters, delta K is a stress intensity factor, and the calculation formula of the Delta K is
Wherein, delta P is the stress amplitude of crack expansion, B is the thickness of the sample, w is the width of the sample in the crack expansion direction, and a is the crack length, the crack expansion rate da/dN value under each cycle number is obtained, and the thermal fatigue crack expansion rate formula of the parent metal welding joint obtained after fitting is da/dN =0.2244 delta K 0.992 ；
Crack growth rate equation da/dN =0.2244 Δ K using thermal fatigue surface 0.992 And obtaining a thermal fatigue surface crack propagation rate curve of the parent metal welding joint, and obtaining a da/dN value corresponding to the initial point of the Paris propagation stage.
In the third step, when the stress intensity factor delta K is calculated, material attributes are defined and the sample is subjected to solid modeling, thermal stress is analyzed by using Ansys software after grids are divided to obtain a thermal structure coupling stress cloud chart of the base material sample, and the maximum thermal stress value is taken as a delta P value.
And defining material attributes to perform solid modeling on the sample, increasing the grid division density at the welding seam and the V-shaped notch of the sample to perform grid division, and acquiring the thermal stress of the tip of the V-shaped notch generated by a temperature place, wherein the maximum thermal stress value corresponds to the tip of the V-shaped notch.
In the third step, when the da/dN value of the crack expansion rate under each cycle number is solved, the measurement record of the surface crack of the corresponding thermal fatigue sample is solved by adopting a secant method, and the da/dN formula solved by the secant method is as follows: da/dN = (a) i+1 -a i )/(N i+1 -N i )。
In the second step, the mode of measuring the surface cracks of the thermal fatigue test sample is as follows: and taking a small section of straight line measurement along the crack propagation path, and averaging after multiple measurements.
And step two, when the thermal fatigue test is carried out on the thermal fatigue testing machine for the thermal fatigue test sample, a natural cooling mode is adopted when the temperature reaches the upper temperature limit.
A method for improving the thermal fatigue resistance of a 409L low-chromium-nickel ferrite stainless steel weld joint is to weld austenitic stainless steel serving as a filling material and add Ce element and Ti element. The mass fractions of the Ce element and the Ti element added were 0.24wt.% and 0.72wt.%, respectively.
CeO with Ce element and Ti element in nano level 2 、TiO 2 Is added in the form of (1). Repeatedly cleaning the to-be-welded section of the welding sample with acetone before welding, weighing the weights of the two welding samples after the acetone is evaporated, and adding CeO 2 、TiO 2 And uniformly coating the acetone solution on the section to be welded, air-drying, weighing the weight of the two welding samples again, and calculating to obtain the welding samples with the mass fractions of Ce and Ti being 0.24wt.% and 0.72wt.%.
Compared with the prior art, the test method for the thermal fatigue surface crack propagation rate of the environment-friendly stainless steel weld joint takes 409L low-chromium-nickel ferrite stainless steel as an example, performs thermal fatigue simulation on the working condition that the environment-friendly stainless steel weld joint is in a cold and hot load needing to experience large temperature difference for multiple times, and can effectively reduce the segregation of Cr element in the ferrite stainless steel, reduce the oxidation rate of the ferrite stainless steel and reduce the occurrence of recrystallization phenomenon by adding Ce element and Ti element to a thermal fatigue test sample before welding, thereby obtaining the conclusion that the addition of Ce element and Ti element during welding of the environment-friendly stainless steel can obtain good thermal fatigue resistance; selecting an exponential model for a thermal fatigue surface crack propagation curve a-N of a ferritic stainless steel welded joint
The resulting thermal fatigue surface crack propagation curve a =206.4e 0.00293N -260.1e -0.01119N The fitting correlation coefficient R value is 0.9968, the fitting precision is high, the relation between the crack growth length and the cycle number is met, and the fitting correlation coefficient R value is better than a = G 1 +G 2 The N linear model and the test data are more accurate; the experimental surface crack propagation rate equation is da/dN =0.2244 delta K after the thermal fatigue crack propagation enters the stable propagation stage 0.992 The same test conditions are used for actual production verification, and the surface obtained by calculation of the equation is measuredThe crack propagation rate is consistent with the actual test, and reliable data support can be provided for the actual production.
Drawings
FIG. 1-1 is a schematic structural view of a thermal fatigue test specimen;
FIG. 1-2 is an enlarged view taken from the direction I of FIG. 1-1;
FIG. 2 is a schematic view of a sample placement location;
FIG. 3 is a graph showing the temperature change of a sample;
FIG. 4 is a macroscopic view of a weld of a sample, wherein (a) is a sample of a base material and (b) is a sample containing rare earth;
FIG. 5 is a macroscopic view of the fracture of a sample, wherein (a) is a parent material sample and (b) is a rare earth-containing sample;
FIG. 6 is a plot of fracture microstructure and surface-scan energy spectra of a parent material sample, wherein (a) is the fracture oxidation defect microstructure, (b) is the spongy oxide microstructure, (c) is the locally enlarged oxidation puddle and crack propagation zone, and (d) is the defect-free oxidation zone elemental composition and distribution;
FIG. 7 is a graphical representation of the microstructure and surface-scan spectra of a fracture of a rare earth-containing sample, wherein (a) is the microstructure of the oxidized region and the cleaved region, (b) is the microstructure of the oxidized region, (c) is the elemental composition and distribution of the cleaved region, and (d) is the elemental composition and distribution of the oxidized region;
FIG. 8 is a plot of a point-scan spectroscopy analysis of a fracture micro-region, wherein (a) is a parent material sample, (b) is a rare earth-containing sample cleavage region, and (c) is a rare earth-containing sample oxidation region;
FIG. 9 is a fitting graph wherein (a) is a linear model fitting curve and (b) is a digital model fitting curve;
FIG. 10 is a thermally-structurally coupled equivalent stress cloud for a fatigue coupon;
FIG. 11 is a graph of 409L thermal fatigue surface crack growth rate.
Detailed Description
Various high-temperature characteristics of the stainless steel can be improved by adopting different processing technologies, adding different stable elements and the like, and different working environments have different influences on the high-temperature characteristics of the stainless steel. The addition of the stabilizing elements such as rare earth can effectively improve the microstructure of stainless steel so as to reduce the size and volume fraction of inclusions, improve the fatigue crack initiation resistance and improve the intergranular corrosion resistance. Taking 409L ferritic stainless steel welded joint as an example, the invention firstly analyzes the generation mechanism and the expansion condition of the thermal fatigue surface crack of the 409L ferritic stainless steel welded joint under the condition of no external load influence at 50-800 ℃, analyzes the a-N curve chart thereof to explore the relation between the crack length and the cycle number, and then introduces a Paris crack expansion model to solve the 409L welded joint thermal fatigue surface crack expansion equation. Specifically, the method comprises the following steps:
1. sample preparation
The test material was 409L (00 Cr11 Ti) ferritic stainless steel hot-rolled weld test pieces of 105mm by 70mm by 4 mm. And (5) polishing by using sand paper and a file to remove burr corners. When in welding, the two welding test plates are in butt welding, and the joint is an I-shaped welding groove. The chemical composition of 409L ferritic stainless steel plate is shown in Table 1. The welding test uses ER304 austenitic stainless steel with a diameter of 1.6mm as the filler material. The welded joint using austenitic stainless steel as filling material has good physical and chemical properties. The chemical composition of the ER304 austenitic stainless steel is shown in table 2.
TABLE 1 409L ferritic stainless steel chemistry (% by mass, wt.%)
Table 2 chemical composition (mass fraction, wt.%) of er304 austenitic stainless steel
And (3) repeatedly polishing the side to be welded of the welding test plate by using abrasive paper to remove impurities, flash, burrs and the like. And cleaning the surface to be welded by using organic solution acetone, removing oil stains, oxides and dust, and immediately performing a welding test after air drying. And welding the welding test plate by using a TIG-200A type inverter direct current argon arc welding machine.
And cutting the welded steel plate by using a linear cutting machine to obtain a thermal fatigue sample containing the V-shaped notch required by the test. The V-shaped notch is positioned in the middle of the welding line and is used for generating stress concentration and inducing cracks so as to be ready for observing the initiation and the propagation of the thermal fatigue cracks of the micro sample. The center of the linear cutting processing weld joint is coincided with the center of the V-shaped notch of the sample, and the thermal fatigue sample is shown in figures 1-1 and 1-2.
In order to improve the performance of the welded joint of the 409L stainless steel, 14 groups of welded joint samples with different Ce and Ti contents are prepared in a test way, and are subjected to impact and tensile tests, and the test results show that the sample performance is best when the mass fractions of Ce and Ti are respectively 0.24wt.% and 0.72wt.%. Similarly, in this test, ce and Ti are nano-sized CeO 2 、TiO 2 The method comprises the steps of repeatedly cleaning a to-be-welded section of a 409L welding test plate by using acetone before welding, and weighing the weight of the two welding test plates after the acetone is evaporated. Will contain CeO 2 、TiO 2 And uniformly coating an acetone solution on the section to be welded, air-drying, weighing the weight of the two welding test plates again, and calculating to obtain a group of welding test plates with the mass fractions of Ce and Ti respectively being 0.24wt.% and 0.72wt.%. The 409L welding joint which contains Ce and Ti elements and has good performance is obtained by adopting the same welding process and cutting mode.
2. Thermal fatigue test
The thermal fatigue test is carried out on a DST-01 thermal fatigue testing machine, a micro thermal fatigue sample containing Ce and Ti elements is separated from a micro thermal fatigue sample of a parent metal by a quartz plate, an induction thermocouple is placed at the center of a welding seam of the sample, the sample placing position is positioned at the center of an induction coil, the sample is uniformly heated, and the sample placing position is shown in figure 2.
Before the test is started, cooling water is started, the upper limit and the lower limit of the temperature are set to be 800 ℃ and 50 ℃, the power box is adjusted, the temperature rise time is set to be 161s, when the temperature reaches 800 ℃, a natural cooling mode is adopted, the average cooling time is 500s, and the temperature change curve of the sample is shown in fig. 3.
And (4) carrying out low-temperature impact on the sample after the test along the center of the welding seam, and cleaning the impacted fracture by using ultrasonic waves to remove impurities and oxides on the surface.
3. Fracture tissue analysis and energy spectroscopy
And (5) performing tissue analysis on the fracture of the sample by adopting a scanning electron microscope. And in order to assist in judging the chemical components of the fracture, an energy spectrum analyzer is adopted to carry out energy spectrum analysis on the fracture.
After 1100 times of high-low temperature cycles of 50-800 ℃, the two thermal fatigue samples are deformed in different degrees, and a large number of net-shaped cracks exist on the surfaces. The micro thermal fatigue test sample added with Ce and Ti elements has relatively small overall deformation, but at the welding seam, because the thermal expansion coefficients of 409L ferritic stainless steel and ER304 austenitic stainless steel of the welding seam filling material are different, deformation occurs at the welding seam, the reticular cracks of the welding seam surface of the test sample are less, and no cracks occur at the V-shaped notch and the fusion zone, as shown in figure 4 (b). Compared with a micro thermal fatigue sample added with Ce and Ti elements, the base material sample has larger overall deformation, more reticular cracks appear around the welding seam, the width and depth of the cracks are larger, a large amount of fine cavities and a small amount of falling-off appear at the welding seam, so that the welding seam presents a spongy appearance, visible cracks exist at the V-shaped gap, the cracks expand to the fusion area and continue to expand downwards along the boundary direction, as shown in figure 4 (a). From this, it can be seen that the micro thermal fatigue test piece added with Ce and Ti elements exhibits better thermal fatigue resistance than the micro thermal fatigue test piece of the base material.
Observing the macro morphology of the fractures of the two samples, it can be seen that the thermal fatigue test sample of the parent material has serious fracture oxidation phenomenon, which is represented by a large amount of black oxygen attached to the surface of the fracture, the oxide is loose and easy to fall off, and the fracture has serious overall deformation, as shown in fig. 5 (a). The micro thermal fatigue test sample added with Ce and Ti only has oxidation phenomena at the edges at two sides of a fracture, the oxide is compact compared with the fracture of the parent metal and is not easy to fall off, the center position of the welding seam of the test sample has no oxidation phenomenon, and the deformation quantity of the fracture is relatively small, as shown in fig. 5 (b).
Observing the microscopic appearance of the fractures of the two samples, as shown in fig. 6 (a) and 6 (b), it can be found that the appearance of the fractures of the parent metal sample is uneven, a large oxidation hollow exists, a thick and loose oxidation layer is distributed around the hollow, a plurality of micropores exist on the oxidation layer, and the whole body presents a loose and easily-falling spongy shape. In addition, there are dense thermal fatigue cracks at the fracture surface, with larger cracks propagating along paths where micro-pores and oxidation defects are more severe. As shown in fig. 7 (a), the fracture of the rare earth-containing sample can be found to have an obvious oxidation zone and a cleavage zone, most of the cleavage zone is in the shape of a river flower consisting of cleavage steps, a tongue-shaped cleavage step exists, an obvious slip band exists on the cleavage step, a protruding tearing ridge exists, and the fracture shows a mixed fracture in which brittleness and toughness exist simultaneously; the oxidation region is enlarged locally to obtain fig. 7 (b), and it is found that there are significant oxidation cracks and oxidation streaks in the region, and many fine particles are distributed. The oxidized region of the rare earth-containing sample does not exhibit a loose sponge-like but a more dense oxide layer than the oxidized region of the base material sample.
FIG. 6 (c) is a partially enlarged view of a region where the base material cracks are converged and the oxidation defects are large. In this region, there are relatively dense fine cracks, which intersect each other and are the main cause of oxidation pits due to falling off. However, the main reason for inducing the generation of small cracks and micro-pores is that the weld joint has a metal heat activation recovery phenomenon under high temperature conditions. In the whole temperature rise process of the sample at 50-800 ℃, the metal in the sample is converted into dislocation motion at high temperature from the mechanism of vacancy motion at low temperature and combination of vacancies and other defects, and the dislocation motion ensures that the randomly distributed dislocations in the original deformation body are redistributed and arranged to a low energy state to form subgrain. Along with the increase of the cycle number, the vacancy motion and the dislocation motion are continuously increased, fine cavities and small cracks are finally formed, the small cavities are continuously gathered and grow to form a spongy appearance at the welding seam of the base material sample, and the small cracks are connected with the small cracks and the cavities continuously expand to form large cracks. Meanwhile, 409L stainless steel and ER304 stainless steel exist in the welding seam fusion zone, while the dislocation energy of ER304 austenitic stainless steel is relatively low, so that the propagation of surface cracks is easier under the high-temperature condition according to the characteristics of recovered dislocation motion. Furthermore, since there is a large difference in thermal expansion coefficient between two stainless steels, the crack is accelerated by the unequal amount of deformation caused at high temperature, and thus the crack generated at the V-notch is propagated along the fused region as shown in fig. 4 (a). In addition, the presence of brittle oxides attached to the fracture accelerates cyclic slippage, causes local transgranular cracking, and accelerates crack propagation. In conclusion, thermal defects caused by vacancy movement form micropores, and finally an oxidation zone with a spongy morphology is formed; while the strengthening effect of solute atoms at high temperature promotes the plane slip of dislocation and the formation of slip band as in fig. 6 (a), and finally generates fine cracks; and the crack propagation is the result of the combined action of vacancy motion, dislocation motion, unequal deformation and high-temperature oxidation.
The thermal fatigue test sample added with Ce and Ti elements has good thermal fatigue resistance, and the oxide generated under the same test condition is more compact and less prone to fall off.
As a result of performing a surface-scan energy spectrum analysis of an oxidized region of a fracture of the base material, which is free from a large defect and is located on the left side of the dotted line in FIG. 6 (c), the oxidized region is mainly composed of Fe and Cr elements as shown in FIG. 6 (d). The results of the surface-scan spectral analysis of the oxidized region of the rare earth-containing sample shown in FIG. 7 (b) are shown in FIG. 7 (d), and the main constituent elements of the oxidized region are also Fe and Cr. It was found that the oxides of the two samples had substantially the same composition elements and did not differ greatly in content, but showed distinct differences. The two samples were further analyzed for oxidation area, and in fig. 6 (C), a circular micro area where the base material sample cracks converged and defects are more evident was subjected to a point-scan energy spectrum analysis, and the result is shown in fig. 8 (a), where the micro area is mainly composed of Fe, cr, and C elements. The same point scan spectroscopy analysis was carried out on the defective circular micro-area of the oxide layer of the rare earth-containing sample in FIG. 7 (b), and the result is shown in FIG. 8 (C), which micro-area is mainly composed of Fe, cr, C, si elements. Although the main constituent elements of the two micro-regions both contain Fe, cr and C, the Fe element content of the rare earth-containing sample is nearly twice that of the parent material sample, the C element content is far less than that of the parent material sample, and the Cr element content is far less than that of the parent material sampleLess elements than the parent material sample. It is inferred that CrC compounds and (Fe, cr) may be generated at a portion where the crack defect of the base material sample is conspicuous under high temperature conditions 2 O 3 And thermally oxidizing the film. CrC compound causes Cr element at the fracture of the parent metal sample to segregate, and the decrease of Cr element causes a significant decrease in the corrosion resistance of stainless steel and damages (Fe, cr) 2 O 3 The film is thermally oxidized, accelerating the oxidation behavior. The rare earth-containing samples likewise produced CrC compounds and (Fe, cr) 2 O 3 The film is thermally oxidized, but the content of CrC should be much less than that of the base material. Because the added Ti element has stronger affinity with the C element, the Ti element preferentially reacts with the C element to generate a TiC compound, and the defects that the steel stability is damaged and the Cr element is lost by non-metallic inclusions containing C and N are avoided. However, the presence of Cr produces a dense thermal oxide film (Fe, cr) with Fe 2 O 3 And the loss of Cr element is slowed down, and the function of protecting the welding seam is achieved. The existence of the Si element is beneficial to the high-temperature oxidation resistance of the stainless steel. It can be seen that the addition of Ti reduces the loss of Cr element and prevents the destruction of (Fe, cr) 2 O 3 Formation of a large amount of CrC dense in the thermal oxide film. Meanwhile, the addition of the Ce element is beneficial to the densification and uniformity of an oxide film, and can obviously reduce the oxidation rate and increase the oxidation quality.
Although under the same test conditions, the deformation of the weld of the two samples is very different. For this reason, the cleavage area of the rare earth-containing sample in FIG. 7 (a) was swept, and as a result, as shown in FIG. 7 (c), the rare earth-containing sample was mainly composed of Fe and Cr elements and the elements were uniformly distributed. However, the surface scanning spectrum analysis is often used as an aid to judge the distribution of elements, and the determination of the element content is not so accurate. Therefore, in FIG. 7 (a), the micro-area of the cleavage area near the edge of the oxidized area is analyzed by the point-scan spectroscopy, and the result is shown in FIG. 8 (b). The Fe and Cr elements in the area can be found to be respectively as high as 86.56 percent and 9.13 percent. The area has higher content of Fe and Cr elements, which shows that the area has good mechanical property. The reason is that the added Ce element can refine grains, reduce the size of the grains, delay the inward diffusion of oxygen element, generate the effects of solid solution strengthening and improving grain boundaries, hinder alloy deformation dislocation and relieve high-temperature deformation of welding seams.
Namely, the addition of Ce and Ti can effectively reduce the segregation of Cr element in the ferritic stainless steel, reduce the oxidation rate and reduce the occurrence of recrystallization.
4. Calculation of thermal fatigue surface crack propagation rate of welded joint
The 409L ferrite base material sample was subjected to thermal fatigue tests 50, 100, 200, 300, 400, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, and 1100 times, and then the surface crack length was measured. The measurement mode is as follows: and taking a small section of straight line measurement along the crack propagation path, and averaging after multiple measurements. The measurement results are shown in table 3.
TABLE 3 surface crack Length (. Mu.m) at different number of cycles (N)
Fitting the table 3, namely 18 test data points by using Matlab software, taking the crack length a as a crack length value of a fitting curve corresponding to a single test index, and taking the cycle number N as a cycle number value corresponding to a single test variable. Selection of a commonly used linear model a = G for a thermal fatigue crack length fitting model 1 +G 2 N, wherein a is the crack length, G 1 、G 2 And N is the cycle number for the coefficient to be solved. The final fitting results are shown in fig. 9 (a), and it is clear that the linear model is not suitable for the thermal fatigue surface crack propagation of this test. The coincidence degree of the tested data points and the fitting straight line is not ideal, and the dispersion degree is high, so that the crack propagation rate and the cycle number obviously do not show a linear relation. Selection of an exponential model
Refitting 18 experimental data points, where C 1 、C 2 、C 3 、C 4 The coefficient to be solved is an a-N equation exponential model solved after fitting according to the crack lengths corresponding to different cycle times, and the fitting result is shown in FIG. 9 (b)As shown, the 18 test points are uniformly distributed around the a-N curve fitted by the exponential equation. And the fitting correlation coefficient R value is 0.9968, the fitting precision is high, and the relation between the crack growth length and the cycle number is met. Obtaining a-N curve equation a =206.4e 0.00293N -260.1e -0.01119N 。
Introducing a Paris formula for researching the change rule of the thermal fatigue surface crack propagation rate of the welding joint in the medium-speed propagation stage. In the Paris expansion stage, the conditions of (beta = a/w, wherein beta is more than or equal to 0.3 and less than or equal to 0.9, a is the crack expansion length, and w is the width of the sample in the crack expansion direction) need to be met, so that the crack lengths corresponding to 900, 950, 1000, 1050 and 1100 cycles are taken as a sample in the test.
In order to obtain the equivalent stress generated by the base material sample in the process of 50-800 ℃ under the test condition, the test is subjected to thermal structure coupling, and the thermal stress is analyzed by using Ansys software to obtain the magnitude of the thermal stress generated by the V-shaped notch tip from a temperature place. The actual operating heating of the thermal analog fatigue machine is shown in fig. 3. The high temperature material parameters for 409L ferritic stainless steel and ER304 austenitic stainless steel are shown in Table 4.
TABLE 4 Material parameter Table
Defining material attributes, carrying out solid modeling on the sample, and properly increasing the grid division density at the welding seam and the V-shaped notch of the sample during grid division to finally obtain a thermal structure coupling stress cloud chart (as shown in figure 10) of the base material sample. The maximum thermal stress of the V-port tip of the sample was found to be 61.86MPa by stress cloud. It is known from the mechanism of fatigue crack generation that a fatigue crack source is relatively easily generated when stress concentration occurs at a certain point of a specimen. For the sample of the test, stress concentration is easy to generate at the tip of the V-shaped notch, a fatigue source is easy to generate, and the same is true for the actual test. When the stress intensity factor delta K is calculated, the stress value of the tip of the V-shaped notch is 61.86MPa according to the stress amplitude delta P.
Therefore, a Paris formula is introduced to solve a propagation rate formula of the thermal fatigue surface crack at the stable propagation stage of the base material sample.
Formula of Paris equation: da/dN = C (. DELTA.K) m Wherein a represents the crack length, N represents the cycle number, C and m are material parameters, and DeltaK is a stress intensity factor.
The formula of delta K is
Wherein, deltaP is the stress amplitude of crack propagation, B is the thickness of the sample, w is the sample width in the crack propagation direction, and a is the crack length.
(1) Solving the crack propagation stress amplitude DeltaP:
the thermal stress generated at the weld joint with the maximum stress of 800 ℃ obtained by the previous Ansys simulation is coupled to 61.86MPa in the structural stress, and the thermal stress generated at 50 ℃ is almost ignored, so that the delta P of the finally obtained crack tip is 61.86MPa.
(2) Solving the stress intensity factor delta K:
the obtained crack propagation stress amplitude Δ P is substituted into a stress intensity factor formula, and finally the stress intensity factors under different cycle times are obtained as shown in table 5.
(3) Solving the crack propagation rate da/dN at each cycle number:
and (3) corresponding the crack length a measured by the test for 900-1100 times to the data in the table 3, and solving by adopting a secant method or a method of increasing a polynomial to obtain the corresponding da/dN value under each cycle time. The secant method is adapted to calculate the slope of a straight line from data points adjacent to an area on the a-N curve, and the increasing polynomial is derived using a local fit to determine fitted values for fatigue crack propagation rate and crack length. For any test data point i, namely each N point before and after, (2N + 1) continuous data points. For the few data points in the table above, the secant method is used to solve the data points, and the values of da/dN at different cycle times are obtained as shown in table 5. Solving the da/dN formula by a secant method: da/dN = (a) i+1 -a i )/(N i+1 -N i )。
TABLE 5 stress intensity factor Δ K and crack propagation rate da/dN
Fitting the corresponding stress intensity factor delta K (x) and da/dN (y) values in the table 5 to finally obtain a thermal fatigue surface crack propagation rate formula of the base metal welding joint: da/dN = 0.2244. Delta.K 0.992 。
(4) Drawing of thermal fatigue crack propagation rate curve chart of 409L welding joint
Crack growth rate equation da/dN =0.2244 Δ K using thermal fatigue surface 0.992 A 409L weld joint thermal fatigue surface crack propagation rate curve as shown in fig. 11 was obtained. Paris expansion phase starting point corresponds to an approximate Δ K of 39.1267MPa · m 1/2 At this time, the da/dN value was 8.0734X 10 -4 mm/cyc。
In summary, a =206.4e for the thermal fatigue surface crack propagation curve a-N for a 409L ferritic stainless steel weld joint 0.00293N -260.1e -0.01119N The model is better than a = G 1 +G 2 N model; when thermal fatigue reaches 912 times, the crack expands into a stable expansion stage, and the expansion rate can be increased by da/dN =0.2244 delta K 0.992 And (4) expressing an equation. The same test conditions are used for carrying out actual production verification, and the surface crack propagation rate calculated by the equation is in accordance with the actual test through measurement, so that reliable data support can be provided for actual production.
Claims (5)
1. The test method for the crack propagation rate of the thermal fatigue surface of the environment-friendly stainless steel weld joint is characterized by comprising the following steps of:
step one, sample preparation: using austenitic stainless steel as a filling material, carrying out butt welding on two 409L low-chromium-nickel ferrite stainless steel samples, and then cutting to ensure that the center of a cutting weld seam coincides with the center of a V-shaped notch of the sample, thereby obtaining a thermal fatigue sample containing the V-shaped notch, which is required by a test;
step two, thermal fatigue test: performing thermal fatigue test on a thermal fatigue testing machine according to the set test times, wherein a natural cooling mode is adopted when the temperature reaches the upper limit of the temperature during the thermal fatigue test, and the surface cracks of the thermal fatigue test sample are measured and recorded when the set test time data sampling points are reached;
step three, calculating the thermal fatigue surface crack propagation rate of the welding joint: selecting an exponential model
Wherein C is 1 、C 2 、C 3 、C 4 Taking the crack length a as the crack length value of a fitting curve corresponding to a single test index, taking the cycle number N as the cycle number value corresponding to a single test variable, and fitting a plurality of surface crack length data points acquired by test number data sampling points by using Matlab software to obtain an a-N surface crack propagation curve equation a =206.4e 0.00293N -260.1e -0.01119N ；
Introduces Paris formula da/dN = C (delta K) m Solving a formula of the expansion rate of the base material sample at the thermal fatigue surface crack stable expansion stage, wherein a represents the crack length, N represents the cycle number, C and m are material parameters, delta K is a stress intensity factor, and a calculation formula of the delta K is
Wherein, delta P is the stress amplitude of crack expansion, B is the thickness of the sample, w is the width of the sample in the crack expansion direction, and a is the crack length, the crack expansion rate da/dN value under each cycle number is obtained, and the thermal fatigue crack expansion rate formula of the parent metal welding joint obtained after fitting is da/dN =0.2244 delta K 0.992 ；
Crack growth rate equation da/dN =0.2244 Δ K using thermal fatigue surface 0.992 And obtaining a thermal fatigue surface crack propagation rate curve of the parent metal welding joint, and obtaining a da/dN value corresponding to the initial point of the Paris propagation stage.
2. The method for testing the thermal fatigue surface crack propagation rate of the environmental stainless steel weld according to claim 1, wherein in the third step, when the stress intensity factor Δ K is calculated, material attributes are defined and a sample is subjected to solid modeling, thermal stress is analyzed by using Ansys software after grids are divided to obtain a thermal structure coupling stress cloud chart of a base material sample, and the maximum thermal stress value is taken as the Δ P value.
3. The method for testing the thermal fatigue surface crack propagation rate of the environmental-friendly stainless steel weld according to claim 2, characterized in that when material attributes are defined to perform solid modeling on a sample, the meshing density of the weld joint and the V-shaped notch of the sample is increased to perform meshing, the thermal stress generated by the temperature field at the tip of the V-shaped notch is obtained, and the maximum thermal stress value corresponds to the tip of the V-shaped notch.
4. The method for testing the thermal fatigue surface crack propagation rate of the environmental-friendly stainless steel weld joint according to claim 1, wherein in the third step, when the da/dN value of the crack propagation rate under each cycle number is solved, the measurement record of the surface crack of the corresponding thermal fatigue test sample is solved by adopting a secant method, and the da/dN formula solved by the secant method is as follows: da/dN = (a) i+1 -a i )/(N i+1 -N i )。
5. The method for testing the thermal fatigue surface crack propagation rate of the environmental-friendly stainless steel welding seam according to claim 1, wherein in the second step, the surface crack of the thermal fatigue test sample is measured by the following method: and taking a small section of straight line measurement along the crack propagation path, and averaging after multiple measurements.
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